Role of relaxation in the quantum measurement of a superconducting qubit using a nonlinear oscillator

TL;DR

This paper investigates how relaxation affects the measurement of a superconducting flux qubit using a nonlinear oscillator as a quantum non-demolition detector, introducing a model to analyze the process and achieving high readout fidelity.

Contribution

It presents a simple transition rate model for qubit relaxation and detector switching during measurement, highlighting the dependence on driving strength and oscillator state.

Findings

Readout fidelity exceeds 95% after correction for relaxation

Qubit relaxation varies significantly with driving strength

Detector switching behavior is characterized by the transition rate model

Abstract

We analyze the relaxation of a superconducting flux qubit during measurement. The qubit state is measured with a nonlinear oscillator driven across the threshold of bifurcation, acting as a switching dispersive detector. This readout scheme is of quantum non-demolition type. Two successive readouts are used to analyze the evolution of the qubit and the detector during the measurement. We introduce a simple transition rate model to characterize the qubit relaxation and the detector switching process. Corrected for qubit relaxation the readout fidelity is at least 95%. Qubit relaxation strongly depends on the driving strength and the state of the oscillator.